1. Field of the Invention
The present invention relates to backlight units that illuminate transmissive liquid crystal panels and to liquid crystal display devices equipped with such backlight units.
2. Description of the Related Art
Liquid crystal display devices optically display, for example, predetermined images by applying voltage to liquid crystal sealed between two transparent substrates. In detail, the application of voltage causes the orientation of the liquid crystal molecules to change, thereby changing the light transmittance. A liquid crystal display device is equipped with a backlight unit that illuminates a transmissive liquid crystal panel included in the device.
A backlight unit is generally equipped with a cold cathode fluorescent lamp (CCFL) or light-emitting diodes (LEDs) as a light source. Because the light crystal itself is not self-active, the backlight unit emits illumination light evenly and stably over the entire surface of the liquid crystal panel from the rear side of the panel so that a predetermined image, for example, can be displayed on the liquid crystal panel.
FIG. 15 shows a backlight unit 100 disclosed in Japanese Unexamined Patent Application Publication No. 2003-281924. In this backlight unit 100, a light guide plate 103 has a side surface 103a to which a light-source substrate 102 is attached, and the light-source substrate 102 has mounted thereon a plurality of LEDs 101 as a light source in a substantially single line. The LEDs 101 are used in place of a CCFL since they consume less power, are smaller in size, are lighter in weight, and allow for easier maintenance.
In the backlight unit 100 disclosed in Japanese Unexamined Patent Application Publication No. 2003-281924, illumination light is emitted from the LEDs 101 in a diffusing manner so as to enter the light guide plate 103. The illumination light undergoes total reflection repetitively within the light guide plate 103 so that the illumination light can be widely distributed to the far side of the light guide plate 103. In order for the illumination light to undergo total reflection within the light guide plate 103, it is preferred that an incidence angle θ1 of the illumination light incident on the light guide plate 103 be equal to or above a critical angle.
In this case, an incidence angle θ1 of illumination light refers to an angle formed between a ray of illumination light and a normal line of a boundary surface of the light guide plate 103 at a point where the illumination light in the light guide plate 103 and the boundary surface of the light guide plate 103 intersect.
The critical angle can be determined on the basis of Snell's law.sin θ=nA/nB(nB>nA)  Formula 1
where “θ” represents a critical angle, “nA” represents a refractive index of medium A, and “nB” represents a refractive index of medium B.
In the backlight unit 100 disclosed in Japanese Unexamined Patent Application Publication No. 2003-281924, supposing that the light guide plate 103 composed of an acrylic material has a refractive index of about 1.5 and the refractive index of air is about 1.0, the critical angle can be determined to be about 41.8° based on Snell's law. As shown in FIG. 15, regarding illumination light propagating from the acrylic light guide plate 103 to air, the incidence angle θ1 of the illumination light is preferably about 41.8° or higher in order to allow the illumination light to undergo total reflection at the boundary surface of the light guide plate 103.